The present invention is related to tissue surveillance systems.
Systems and methods exist for determining when a probe, needle, catheter or other devices make contact with a particular tissue, e.g., U.S. Pat. No. 5,836,990 to Li entitled xe2x80x9cMethod and Apparatus for Determining Electrode/Tissue Contactxe2x80x9d. The Li patent teaches a method for determining when a catheter makes contact with tissue covered with an ionic liquid. The system measures the electrical impedance at a distal end of the catheter and determines tissue contact has been made when the impedance increases. The system does not identify the type of tissue contacted and presumes the tissue is covered in an ionic liquid. Accordingly, a need exists for a system and method that identify tissue and use this information in medical procedures.
Systems and method also exist for controlling the level of ablation of tissue. These systems monitor the impedance of tissue being ablated to determine if the ablation energy is optimal. The systems generally measure impedance to within approximately 20 ohms. These systems do not determine when sufficient therapy has been applied to the tissue and employ impedance measurement with low tolerance levels. Accordingly, a need exists for a system that may control any form of therapy by monitoring characteristics of an electrical signal applied to the tissue.
The present invention provides a system in which an electrical signal is applied to a tissue via electrodes disposed on a tissue probe. The electrical signal applied to the tissue preferably comprises a frequency variable current or voltage that is preferably applied to the tissue using a sliding frequency scale.
In accordance with the present invention, the response to the applied signal is measured as the signal passes through tissue disposed at, around, or adjacent to, the probe. The inventors have found that different tissue types display different electrical transmission properties, including different capacitance and impedance properties. Accordingly, by measuring the electrical characteristics of the response signal, it is possible to determine the type of tissue through which the signal is passing. Preferably, this is accomplished by comparison to known exemplary signal characteristics for various tissue types. Further, when the probe is known to be a first tissue, the system and method may determine when the probe is advanced into a different tissue based on the changed electrical characteristics of the signal applied the probe.
In accordance with the present invention, the electrical signal characteristics that are monitored may include the phase shift between the voltage and current passing through a selected tissue, and the impedance of the selected tissue. The present inventors have experimentally determined that these properties vary from one tissue type to another. In a preferred aspect of the present invention, the electrical signal applied to the tissue may be a sliding frequency signal so a frequency spectrum of phase shift and impedance of a tissue is determined, however, any electrical, magnetic, or optical signal whose phase relationship and impedance to passage through the tissue may be measured can be used.
In a preferred method, a probe is advanced to a position in, at, or adjacent to, a selected tissue and an electrical signal is applied to the tissue by an electrode on the probe. The response to this signal is then measured and compared against electrical, magnetic, or optical transmission characteristics for the various tissue types. For example, the present invention provides a method and system for determining whether the conductive tip of a pedicle probe or pedicle screw is located in one of cortical bone, cancellous bone, and cortical bone near a boundary with soft tissue, whether the conductive tip of a cannula is located adjacent to one of nerve tissue and annulus tissue, and whether the conductive tip of a cathode is located adjacent to one of nerve tissue and prostate gland tissue.
Further, the inventors have discovered that the signal transmission characteristics of various tissues vary as a function of the tissue""s health. Accordingly, the present system can also be used to determine tissue health (for various tissue types) by comparing the signal responses of tissue (in response to stimulation by the probe) to responses for healthy tissue.
The present inventors have determined that different cell/tissue types exhibit different capacitive effects. In addition, these capacitive effects vary considerably between living and dead cells. Accordingly in another aspect of the invention, the present system discriminates between living and dead tissues. This feature of the invention is particularly useful when the present system is used in conjunction with a tissue ablation system. For instance, the tissue ablation system may be prevented from providing unnecessary energy to ablate tissue and thereby protect surrounding tissue.
Moreover, the present system can be adapted to sense the presence of a particular type(s) of tissue as the probe is advanced through the patient""s body. Such a feature of the present invention is particularly advantageous when sensing for the presence of nerve tissue. Specifically, the probe can be advanced through the patient""s body, with the response to the electrical stimulation emitted by the probe being continuously monitored such that as nerve tissue is approached; the response signal will begin to exhibit characteristics indicative of nerve tissue.
Such nerve sensing features of the present invention can be used, for example, to sense for the presence of spinal nerves when advancing surgical equipment (which may include cutting, drilling, screw insertion, implant, and tissue ablation systems) towards the patient""s intervertebral space.
In an optional aspect of the present invention, a probe having an electrode positioned thereon is replaced with a probe, which is itself electrified. For example, an electrified needle or an electrified trocar or cannula can be used as the probe. An advantage of having the entire probe emit the signal (rather than just an electrode disposed thereon) is that the probe itself can be made to smaller dimensions, particularly in the case of an electrified needle.
In optional aspects of the present invention, the probe is mono-polar. Specifically, only a first electrode is disposed on the probe. A second electrode is then positioned some distance away from the first electrode at another location on the body. Alternately, the probe may be bi-polar with both the first and second electrodes positioned on the probe itself. Additionally, the probe may include a plurality of bi-polar electrodes placed along the probe (such as around the tip and the length of the probe) to determine tissue types around the probe.
In a preferred aspect of the present invention, the measurement of the phase angle relationship between the voltage and current of the signal and impedance of the signal may be used to determine: (1) the type of tissue in which the probe is located, (2) the health of the tissue, (3) the relative location of the tip of the probe (ie: in cases where the electrode is disposed in the tip of the probe); and (4) any combination of (1), (2) and (3). As such, by gathering data mapped by analyzing the response signal, measured characteristics can be used to correlate: (1) tissue identity, (2) tissue health, and (3) tissue location.
In addition, the present invention can be adapted to: (5) locate specific tissue within a body; (6) control application of therapy to tissue; (7) detect the state of health of tissue; (8) navigate to tissue; and (9) any combination of the above.
In one embodiment, the invention is a tissue system including a computer system having an analog to digital (A/D) converter and digital to analog (D/A) converter interface (PCI board), that may be used to generate the control signal which is applied to the electrode or conductive tip of the probe. The computer generates the signal via the D/A converter. Then the A/D converter converts the signal received from the conductive tip into digital samples by sampling the signal at a predetermined rate where the digital samples may have a fixed or variable number of bits and have linear, logarithmic or other scaling. The computer system determines characteristics of the received signal from the digital samples, in particular the phase angle and impedance at the conductive tip or other location of the probe where the electrode(s) may be located. Based on the determined characteristics taken over time (which is then stored in a knowledge base or tabulated form), the present invention may determine tissue identity and tissue location. In a preferred aspect, the electrode disposed on the probe comprises a bipolar electrode conductive tip probe.
In an optional aspect of the present invention, the application of therapy to the tissue in which the probe is located may be precisely controlled. Based on the characteristics of the tissue where the probe is located, tissue therapy application may be precisely controlled. For example, the application of heat or cooling therapy may be used to ablate or cool tissue. In one exemplary aspect, the same electrode(s) used for tissue discrimination (ie: determining tissue type for tissue disposed adjacent to the electrode on the probe) may also be used for tissue ablation by heating.
In various aspects, the level of heating or cooling of the tissue may be modulated as a function of the measured characteristics of the tissue. In particular, the phase angle and impedance of the tissue change as the tissue is heated or cooled to certain level. Accordingly, the application of therapy may be regulated by the present computer system. In particular, the computer system may communicate with a device applying therapy and automatically control the level of therapy.
Given that the present system can determine the type and location of various tissues within a patient, the present system may be used to determine the relative health of the tissue. In particular, the measured characteristics of the signal will vary for diseased or unhealthy tissue, as compared to normal healthy tissue. Thus, the present system may be used to determine the type of tissue, the location of the tissue, the health of tissue, and also to control therapy for tissue based on the same. Furthermore, the probe may optionally be coupled with an automated navigation system that navigates within the patient based on the measured characteristics of the received signal. Such a navigation system may use the tissue identity and location data to navigate to a particular location within an organ. Then the computer system may determine the health of the tissue at the location within the organ and control the application of therapy as appropriate.
As can be envisioned by one of skill in the art, many different combinations of the above may be used and accordingly the present invention is not limited by the scope of the appended claims.
In optional aspects of the invention, the characteristic electrical properties of the various tissue types are determined for different tissues at different RF frequencies. For example, the signal may be emitted from the probe (into the surrounding tissue) at frequencies in the range of 400 kHz to 100 MHz. Determining the electrical properties of various tissues at various signal frequencies may be advantageous in that different cell (ie: tissue) types may exhibit different harmonics. As such, tissues may be further characterized by measuring phase shift or impedance at various frequencies, or along a sliding frequency.